CNC Machining Metals: A Machinist’s A-Z Guide

Opening War Story: The Bolt That Broke the Budget

A young aerospace engineer, brilliant and ambitious, walked into our RM factory a few years ago. He was designing a new type of high-performance drone, and he handed me a beautiful 3D model of a custom shoulder bolt. It was a simple-looking part, about two inches long. I need 200 of these,” he said, “The material is specified on the print: Inconel 718.

I looked at the print, then at the engineer. “Are you sure?” I asked. “This is for a drone prototype, right? Not the inside of a jet engine?”

He nodded. “We need extreme heat resistance for the motor mount. It’s non-negotiable.”

A senior machinist working nearby overheard the conversation and chuckled. He walked over, picked up a similar-looking bolt from his toolbox made of 4140 alloy steel, and tossed it in his hand. “This steel bolt?” he said. “We could run 200 of these for you in a day. Maybe a few grand. That Inconel bolt? That’s not a bolt. That’s a week-long nightmare that eats carbide for breakfast.”

The engineer was confused. The parts looked nearly identical. How could the cost and time be so different?

We spent the next hour explaining. We showed him how a cutting tool would glide through the 4140 steel, creating a clean, flowing chip. Then we pulled up a video of machining Inconel. The material didn’t form a chip; it formed a glowing, red-hot, abrasive ribbon that clung to the cutting tool, trying to weld itself to the cutting edge. The forces were so high, and the heat so intense, that a brand-new $100 carbide end mill could be rendered useless in minutes. To machine the Inconel bolt, we would have to run the CNC machine at a fraction of the speed, take tiny, careful cuts, and use a constant flood of high-pressure coolant.

The final quote reflected this reality. The 200 steel bolts were quoted at $3,200. The 200 identical Inconel bolts were quoted at $28,500.

The engineer was stunned. He had chosen his material from a spec sheet based on a single property—heat resistance—without understanding its profound implications for manufacturing. This story is the perfect introduction to the world of CNC machining metals. The answer to “What metals can be machined?” is easy. The answer to “What does it take to machine them?” is where the real knowledge lies.

The Fundamentals: What Does “CNC Machining” Actually Mean?

Before we dive into the metals themselves, let’s establish a baseline. What is happening inside that big machine with the window?

CNC (Computer Numerical Control) is the automated control of machining tools. A computer reads a digital design file (G-code) and translates it into precise movements of a cutting tool in multiple directions (axes).

Subtractive Manufacturing: Unlike 3D printing (additive), CNC machining is subtractive. It starts with a solid block or rod of metal (the “stock” or “workpiece”) and systematically carves away material to reveal the final part.

For metals, this is primarily done on two types of machines:

CNC Milling Machines

A CNC mill uses a rotating cutting tool (like an end mill or drill bit) that moves in multiple axes to cut the stationary workpiece.

• 3-Axis Milling: The tool moves in X (left-right), Y (forward-back), and Z (up-down). This is the standard for most prismatic parts.
• 4-Axis & 5-Axis Milling: These machines add rotational axes (A and B). This allows the tool to approach the part from different angles, enabling the creation of complex, contoured surfaces in a single setup. This directly answers the search query, describe how a 4-axis milling machine differs from a 3-axis milling machine. The fourth axis allows the workpiece to rotate, while a fifth axis allows the tool head itself to tilt, fundamentally increasing the geometric complexity that can be machined.

CNC Lathes (Turning)

A CNC lathe rapidly spins the workpiece (typically a round rod) while a stationary cutting tool moves along its length and diameter to create cylindrical, conical, and threaded features. This process is called “turning.”

The Machinist’s Bible: Understanding the Machinability Index

To compare how different metals behave on a CNC machine, machinists use a concept called the Machinability Index or Machinability Rating.

This is a percentage rating that compares the ease of machining a given material to a standard benchmark. The benchmark is AISI 1212 Carbon Steel, which is assigned a rating of 100%.

• A rating > 100% means the material is easier to machine than 1212 steel.
• A rating < 100% means the material is harder to machine than 1212 steel.

This rating isn’t just about hardness. It’s a complex blend of four key factors:

1. Tool Life: How long does a cutting tool last before it dulls or breaks?
2. Surface Finish: How smooth and clean is the resulting cut surface?
3. Cutting Forces: How much power is required to cut the material?
4. Chip Control: Does the material form small, manageable chips that fall away, or long, stringy, dangerous ones that wrap around the tool?

Understanding this index is the key to understanding why an aluminum part is cheap and an Inconel part is expensive.

The Metals: A Deep Dive into Machinable Families

Now, let’s walk through the factory, from the everyday materials to the exotic superalloys, and examine how each behaves under the cutter.

Aluminum Alloys: The Workhorse

Lightweight, corrosion-resistant, and with excellent thermal conductivity, aluminum is often the first choice for prototypes and production parts. It is generally very easy to machine.

• Machinability Index: 150% – 1000%+
• Why it’s Easy: It’s soft, requires low cutting forces, and allows for extremely high cutting speeds (high RPM and feed rates). Heat dissipates quickly, protecting the cutting tool.
• The Challenge: Some softer alloys can be “gummy,” leading to poor chip control and a phenomenon called “built-up edge” (BUE), where material welds itself to the tool tip, ruining the surface finish.

Common Aluminum Alloys:

• 6061-T6 (The All-Rounder): Machinability ~150%. This is arguably the most popular CNC machined material in the world. It offers a fantastic combination of strength, corrosion resistance, and machinability. It produces predictable chips and excellent surface finishes. Applications: Electronic housings, bike frames, structural components.
• 7075-T6 (The Strongman): Machinability ~120%. Significantly stronger and harder than 6061, this is a go-to for high-stress aerospace applications. It’s a bit more abrasive on tools but still machines very well, often producing a better surface finish than 6061. Applications: Aircraft fuselages, high-performance sporting goods.
• 2024-T3 (The Aerospace Ally): Machinability ~110%. Another high-strength alloy, but it contains copper, which makes it “gummier” and more difficult to machine than 7075. Chip control can be a challenge, requiring very sharp tools and specific geometries. Applications: Aircraft wings and structures where fatigue resistance is key.

Steel Alloys: The Backbone of Industry

From cheap mild steel to ultra-hard tool steels, this massive family of iron-carbon alloys is the most widely used structural material. Machinability varies more here than in any other family.

• Machinability Index: 35% – 125%

Common Steel Alloys:

• Low-Carbon Steel (e.g., 1018, A36): Machinability ~70%. Soft, ductile, and inexpensive. Like some aluminums, its softness can lead to gummy chips and BUE. It’s easy to cut, but achieving a high-quality surface finish requires care. Applications: Welded frames, fasteners, general structural use.
• Alloy Steel (e.g., 4140, 4340): Machinability ~60% (in pre-hardened state). This is “medium-carbon” steel with added alloys like chromium and molybdenum for higher strength and toughness. It’s harder than mild steel but produces excellent, predictable chips and beautiful surface finishes. It’s a machinist’s favorite for strong, reliable parts. Applications: Gears, shafts, engine components, fixtures.
• Tool Steel (e.g., A2, D2, O1): Machinability ~35%. These are very high-carbon alloys designed for extreme hardness and wear resistance. They are typically machined in a softer, “annealed” state and then heat-treated to their final hardness. Machining them in their hardened state is extremely difficult and often requires specialized grinding or hard milling techniques. Applications: Dies, molds, cutting tools, punches.

Stainless Steels: The Resistant Ones

This directly answers the common query: Can stainless steel be CNC machined? Yes, absolutely. But it is one of the most challenging common materials due to a specific material property.

• Machinability Index: 30% – 75%
• The #1 Challenge: Work Hardening. When you cut austenitic stainless steel, the pressure and heat from the cutting tool instantly makes the surface you just cut significantly harder. If your tool dwells in one spot or you take too light of a cut, the surface becomes so hard that the tool can no longer cut it, leading to extreme tool wear and failure.
• The Strategy: The key is to be aggressive. Machinists use sharp, positive-rake tools, a slightly lower RPM, and a constant, heavy chip load. You must get the tool under the previously hardened layer and peel it away with each pass.

Common Stainless Steel Alloys:

• 304 & 316 (Austenitic): Machinability ~45%. These are the most common stainless steels, known for their excellent corrosion resistance. They are gummy, tough, and exhibit high work hardening. They require a rigid machine setup and a smart machining strategy but are machined in high volume every day. Applications: Food processing equipment, medical devices, marine hardware.
• 440C (Martensitic): Machinability ~40%. This is a high-carbon stainless that can be heat-treated to a very high hardness, like a tool steel. It’s typically machined in its annealed state. It is less gummy than the 300-series. Applications: Bearings, knife blades, valve components.
• 17-4 PH (Precipitation Hardening): Machinability ~75%. This is a machinist’s favorite stainless steel. It can be machined in a relatively soft state (Condition A) where it cuts beautifully, and then aged at a low temperature to achieve very high strength and hardness. Applications: Aerospace components, high-strength shafts.

Titanium Alloys: The High-Flyer

Known for its incredible strength-to-weight ratio and corrosion resistance, titanium is the darling of the aerospace, military, and medical industries. It is also notoriously difficult to machine.

• Machinability Index: 20% – 40%
• The Challenges:
  1. Poor Thermal Conductivity: Titanium is an excellent insulator. When you cut it, the heat doesn’t go into the chip or the workpiece; it goes directly into the cutting tool, leading to rapid tool failure. High-pressure, through-spindle coolant is essential.
  2. Chemical Reactivity: At high temperatures, titanium chips can weld to the cutting tool.
  3. “Springiness”: Titanium has a low modulus of elasticity, meaning it tends to flex away from the cutter, which can cause chatter and accuracy issues.

Common Titanium Alloys:

• Grade 5 (Ti-6Al-4V): Machinability ~30%. This is the workhorse of the titanium world, accounting for over 50% of all titanium usage. Machining it is a slow, careful process involving specialized tool coatings, low surface speeds, and high feed rates. Applications: Aircraft structural components, jet engine parts, high-performance engine connecting rods, surgical implants.
• Grade 2 (Commercially Pure): Machinability ~40%. Softer and less strong than Grade 5, this grade is easier to machine but still presents the core challenges of heat management. Applications: Chemical processing equipment, heat exchangers.

Copper, Brass, and Bronze: The Red Metals

This family is known for its electrical and thermal conductivity, corrosion resistance, and, in some cases, spectacular machinability.

• Machinability Index: 70% – 400%+

Common Alloys:

• C360 Brass (Free-Machining Brass): Machinability ~400%+. This is the king. It is so easy to machine that its rating is 4x that of the benchmark steel. The small amount of lead in its alloy causes chips to break into tiny, perfect pieces, resulting in fantastic tool life and surface finish. It’s the ideal material for high-volume, complex parts made on automated screw machines. Applications: Fittings, valves, hardware, musical instruments.
• C110 Copper (E-Cu): Machinability ~70%. Pure copper is soft, gummy, and very difficult to get a good surface finish on. It requires extremely sharp tools (often referred to as “super-sheer”) to avoid smearing the material. Applications: Bus bars, electrical contacts, heat sinks.
• C932 Bearing Bronze: Machinability ~80%. This alloy is strong, corrosion-resistant, and has excellent lubricity, making it ideal for wear applications. It machines very well, producing good chips and finishes. Applications: Bushings, bearings, wear plates.

Superalloys & Exotics: The Final Bosses

This category includes materials designed to operate under extreme conditions of heat, pressure, and chemical exposure. They are, without exception, the most difficult and expensive materials to CNC machine.

• Machinability Index: 5% – 15%

Key Examples:

• Inconel (718, 625): Machinability ~8%. This nickel-chromium superalloy is the material from our opening story. It maintains its strength at incredibly high temperatures, making it essential for jet engines and gas turbines. It has extreme work hardening (even worse than stainless steel) and terrible thermal conductivity, creating a perfect storm of machining difficulty. It requires ceramic cutting tools and very specialized machining strategies.
• Magnesium: Machinability ~500%+. Magnesium is extremely easy to machine, similar to brass. However, it presents a massive fire hazard. Magnesium chips and dust are highly flammable and can be ignited by a spark from a dull tool. Machining magnesium requires special oil-based coolants (never water!), impeccable housekeeping, and specific fire suppression systems. It is often forbidden in general-purpose machine shops.
• Tungsten: Machinability ~15%. Extremely dense and hard with a very high melting point. It’s brittle and very abrasive on tools. Machining it is a slow grinding process.

The “Unmachinables”: What Materials Cannot Be CNC Machined?

While almost any metal can be cut, some materials truly cannot be machined with traditional CNC milling or turning.

• Hardened Tool Steels (60+ HRC): Once a steel is hardened to its maximum potential, it is often harder than the carbide cutting tools themselves. In these cases, the “machining” process shifts from cutting to grinding or Electrical Discharge Machining (EDM), which uses electrical sparks to erode the material.
• Ceramics (e.g., Alumina, Zirconia): These materials are incredibly hard and brittle. They will instantly shatter a standard cutting tool. They are shaped using diamond grinding or ultrasonic machining.
• Sintered Carbides (e.g., Tungsten Carbide): The very material cutting tools are made from. It cannot be machined conventionally and requires diamond grinding or EDM.

Beyond the Metal: Design for Manufacturability (DFM)

Choosing the right metal is only half the battle. How you design your part has a massive impact on its final cost. This is the heart of “metal CNC design.”

• Avoid Sharp Internal Corners: A spinning cutting tool is round, so it will always leave a radius in an internal corner. Specifying a perfectly sharp corner is impossible and requires a secondary process like EDM. Always design with a radius larger than the cutting tool you expect to be used.
• Watch Your Wall Thickness: Very thin walls can vibrate or distort during machining, making them difficult to hold to a tight tolerance.
• Generous Tolerances: The tighter the tolerance (the required precision), the more expensive the part. Tight tolerances may require slower cutting, extra finishing passes, and more frequent inspections. Only specify tight tolerances where they are functionally necessary.
• Minimize Setups: Try to design your part so that all features can be accessed from one or two sides. Every time the part has to be removed and re-fixtured in the machine, it adds significant labor cost and potential for error.

Conclusion: The Dialogue Between Designer and Machinist

The world of CNC machinable metals is vast and complex. The answer to “What metals can be machined?” is simple: almost all of them. But the answer to “Which metal should I use?” is a sophisticated engineering decision with profound consequences for cost, performance, and manufacturability.

The engineer with the drone bolt learned that a material property on a spreadsheet doesn’t tell the whole story. The true story is written on the factory floor, in the dialogue between the cutting tool and the workpiece. It’s a story told in sparks and heat, in the shape of a chip, and in the life of a tool. By understanding the principles of machinability, you move beyond simply choosing a material and begin to design parts in harmony with the physical process that creates them—a skill that separates good designers from great ones.

Frequently Asked Questions (FAQ)

1. What types of metals can be used in CNC machines?
A huge variety, including Aluminum alloys (like 6061, 7075), Steel alloys (mild steel, alloy steel, tool steel), Stainless Steels (304, 316, 17-4 PH), Titanium alloys, Brass, Copper, Bronze, and even difficult Superalloys like Inconel.

2. What materials cannot be CNC machined?
Materials that are harder than the cutting tools themselves are generally not machinable by conventional methods. This includes hardened tool steels (above 60 HRC), engineering ceramics, and sintered carbides. These require alternative processes like grinding or EDM.

3. Can stainless steel be CNC machined?
Yes, absolutely. It is machined in high volumes daily. However, it is more difficult than regular steel because it “work hardens,” meaning the material becomes harder as it is being cut, requiring a specific and aggressive machining strategy to prevent tool failure.

4. Can you use CNC on metal?
Yes. CNC machines, particularly mills and lathes, are the primary tools used for precision manufacturing of metal parts across every industry, from aerospace to medical to automotive.

5. Which of the following is not a common type of CNC machine?
While many machines use CNC, the most common types are mills, lathes, routers, and plasma/laser/waterjet cutters. A machine like a “CNC Forge” would not be a common type, as forging is a fundamentally different process from the subtractive cutting that CNC excels at controlling.

6. What is the best material for CNC carving?
For general-purpose carving and prototyping, 6061 Aluminum is often considered the best starting point due to its low cost, excellent machinability, and good strength. For applications requiring extreme ease of machining and high volume, C360 Brass is unmatched.

References and Further Reading

1. Machinery’s Handbook, 31st Edition: The indispensable reference for machinists, engineers, and designers, containing vast data on material properties and machinability. Industrial Press.
2. Sandvik Coromant: A leading tooling manufacturer with extensive technical guides on machining various materials. sandvik.coromant.com/en-gb/knowledge
3. OnlineMetals.com: A supplier with good general descriptions and properties for a wide range of metals. onlinemetals.com/en/guide
4. Harvey Tool Co.: Technical resources on speeds, feeds, and troubleshooting for machining difficult materials. harveytool.com/resources/technical-resources

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